Several bacterial, rickettsial, viral and fungal blood-stream infections activate the blood clotting system causing intravascular deposition of fibrin which seriously complicates the course of disease. The objective of this research is to study the mechanism of activation of the blood clotting system by endotoxin, an active principle of gram-negative bacteria, and staphylococcal protein A, an active principle of pathogenic staphylococci, both known to initiate coagulation in vivo and in vitro. Cellular and humoral factors of human origin will be analyzed using isotopic, immunochemical, enzymatic and cellular fractionation techniques. Since microbial agents interact with target cells such as platelets, polymorphonuclear leukocytes and endothelial cells to unmask or release procoagulant activity, efforts will be made to characterize the process of attachment of microbial products to the "receptors" on the membrane of human target cells. Subsequent membrane changes in human target cells manifest by leakage of cytoplasmic constituents, release of lysosomal enzymes, modulation of activity of membrane marker enzymes and unmasking or release of procoagulant activity will be examined. The role of complement and the effect of cross-reactive antibodies to Enterobacteriaceae on attachment of endotoxin and subsequent membrane changes will be elucidated. Furthermore, work will continue on determining the molecular mechanism of the staphylococcal clumping test devised by us for routine measurement of fibrinogen and fibrin degradation products in serum and other biological fluids. This will involve characterization of the binding site for staphylococci on polypeptide chains of the fibrinogen molecule, the relationship between the binding site and antigenic determinants of fibrinogen, and the nature of binding between staphylococci and fibrinogen or its derivatives.